The carbon monoxide shift reaction is among the most important reactions of the chemical industry. Recently, this chemical reaction has also become important for power stations using fossil fuels. The background to this is the current trend towards low-carbon dioxide power generation in power stations burning fossil fuels. Here, the carbon monoxide shift reaction can be combined with simultaneous removal of the carbon dioxide formed. According to the present-day prior art, three fundamental approaches are possible: precombustion, post-combustion and oxyfuel.
In the precombustion CO2 capture approach, the carbon monoxide has to be converted into carbon dioxide according to the chemical equation of the carbon monoxide shift reaction before the combustion in order to be able to remove the total carbon in the form of carbon dioxide. Here, the major part of the energy of the carbon monoxide is transferred “shifted” to hydrogen which can be used in a gas turbine.
A disadvantage here is that removal of the two products hydrogen and carbon dioxide in the gas phase is energy-intensive.
A combination frequently proposed in the prior art is the precombustion approach with removal of carbon dioxide combined with the carbon monoxide shift reaction in the gas phase. Here, a synthesis gas is “shifted” in admixture with steam over catalysts at temperatures in the range 300-500° C.
The carbon dioxide is subsequently separated off by means of a physical gas scrub, for example a Rectisol scrub, in a second subprocess. Here, the carbon dioxide is absorbed in cooled methanol at about −40° C. Since these low temperatures are necessary to be able to separate off the carbon dioxide to a sufficient extent, a great amount of energy has to be expended for cooling and this reduces the overall efficiency of the power station.
The European patent EP 0 299 995 B1 discloses a process for converting carbon monoxide and water into carbon dioxide and hydrogen. The combination of a carbon monoxide shift reaction with simultaneous removal of the carbon dioxide formed from the combustion gases described therein is realized in the liquid phase. Particular mention may be made here of example No. 6 and also FIG. 2 of the patent text. In this process, water-containing methanol having a water content of about 2% is used as solvent, with the pH thereof being increased by addition of a carbonate such as potassium carbonate. The chemical reactions, which can in each case proceed physically separately, are as follows:CO+CO32−+H2O→HCOO−+HCO3−  (1)2HCO3−→CO2+CO32−+H2O  (2)HCOO−+H2O→HCO3−+H2  (3)
In the first step corresponding to equation (1), carbon monoxide is bound from the gas phase in a solution and separated from the accompanying components of the synthesis gas. Thermal decomposition of the dissolved hydrogencarbonate HCO3− formed according to equations (1) and (3) is subsequently carried out by increasing the temperature to at least 150° C., with carbon dioxide being formed and preferably being completely removed. The reactions corresponding to equations (1), (2) and (3) in each case take place with the same numbering in a reactor. As last step, the formate HCOO− formed according to equation (1) is decomposed according to the equation (3) and hydrogen is formed. Hydrogen formed according to equation (3) is discharged from the process physically separately from carbon dioxide in a further step.
It has been found that the process corresponding to the patent EP 0 299 995 B1 is not satisfactory for the present use.
Thus, for example, the carbon dioxide cannot be separated off sufficiently in a second reactor. Carbon dioxide is instead discharged with part of the hydrogen and of the residual gases. Furthermore, a large proportion of the solvent methanol is lost with the driving-off of the carbon dioxide in the second reactor. This has to be recovered, for example by means of a water scrub with subsequent distillation. However, this requires increased outlays in terms of process engineering and energy.
The lost hydrogen which is entrained in the carbon dioxide stream in the process and is not separated off amounts to about 8%. These losses lead to very uneconomical operation of the power station with carbon dioxide removal by this process.
Based on the equations (1) and (3), a relatively low yield is therefore achieved in the carbon dioxide removal or the isolation of hydrogen since both gases are not separated off in sufficient purity but partly appear at the outlet for the respective other gas.